The present invention relates to an electric power converter apparatus to convert a power source voltage into an AC voltage having an arbitrary frequency according to a reference voltage, and particularly to a voltage control technique in the case where the power source voltage varies in a variable speed operation of an electric motor.
For example, an electric power converter apparatus to drive an electric motor as a related example will be described with reference to FIG. 8. In FIG. 8, a three-phase alternating voltage supplied from a commercial power source 1 is rectified by a diode unit 2 and smoothed by a smoothing capacitor 3, so that it is converted to a DC voltage Vdc. The Vdc is converted into an AC voltage by an inverter unit 4. An output voltage from the inverter unit is then applied to an electric motor 7 to rotate it. Other means for controlling the inverter unit 4, a reference voltage operating unit 8 calculates an output reference voltage V* in response to a desired speed command value by using a detected current IFB and detected voltage VFB supplied from a current detector 5 and voltage detector 6, respectively. Further, a gate pulse generation unit 9 compares, for example, a reference voltage with a carrier waveform to generate a PWM (pulse width modulation) gate pulse, so that switching devices in the inverter unit 4 are subject to on-off control. The output voltage of inverter unit is therefore proportional to a product of the reference voltage value and DC voltage. Here, when the voltage of commercial power source 1 varies, the DC voltage Vdc is varied, and the inverter unit output voltage is affected by the variation to possibly cause a problem such as an excess current. Therefore, Japanese Patent No. 3573028 has been proposed as related art that the reference voltage V* is corrected in response to the variations of power source and Vdc to suppress the variation of the inverter output voltage. This makes the reference voltage value large when the voltage drops, and also makes the reference voltage value small when the voltage rises, thereby suppressing the affection caused by the Vdc variation and attempting to maintain the inverter output voltage constant approximately.